1. Field of the Invention
Diagnosing a patient""s condition is a challenging, time-consuming, and thus costly task for a health care supplier. Typically, condition is diagnosed and classified using electrocardiography or echocardiography testing. For example, evaluation of cardiomyopathy and assessment of ventricular systolic function are commonly undertaken by echocardiographic techniques that permit comprehensive assessment of left ventricular size and function.
2. Description of Related Art
M-mode echocardiography can be used to obtain precise measurements of left ventricular cavity size and wall thickness at end diastole and end systole but requires extensive procedures for preparation and measurement acquisition. Two-dimensional echocardiography is generally required to appropriately position the M-mode beam and to directly measure ventricular size, left ventricular volumes, ejection fraction, myocardial mass, and chamber volumes. A procedure that includes both two-dimensional and M-mode cardiography is generally required to obtain sufficient information to diagnose cardiomyopathy. High precision measurements are possible but require a high degree of care. Left ventricular mass and volume quantitation by echocardiography requires high-quality images, meticulous attention to proper beam orientation, and usage of accurate geometric models to approximate left ventricular shape.
M-mode echocardiography alone can be used to define many indices of global left ventricular function including ejection phase, fractional shortening of the minor axis, and circumferential fiber shortening velocity. However ejection fraction, the most common index, requires derivation using complex algorithms developed for volume determination from M-mode linear dimensions, visually estimated from two-dimensional echocardiographic images, or obtained by quantitative analysis of two-dimensional echocardiographic images. The algorithms vary greatly in complexity. These procedures are suitably accurate for normally-shaped, normally contracting left ventricles, but are deficient and require much more complex analysis for assessment of deformed ventricles with regional wall motion abnormalities, a not uncommon occurrence in myocardial patients.
These known diagnostic procedures are highly time-consuming, complex, and costly, requiring expensive diagnostic equipment and health provider time. What is needed is an accurate, automatic test capability that would allow tracking of myocardial condition outside the clinic.
In one embodiment, an implantable cardiac stimulation device is disclosed that can be configured to sense and accurately quantify an evoked response resulting from an applied stimulation pulse, derive one or more parameters from the detected evoked response, store the one or more parameters over long time periods, and derive variability statistics from the one or more parameters to assist in tracking the patient""s condition over time, which may be used to guide the patient""s therapy.
An implantable cardiac stimulation device uses variation in cardiac evoked response as an indicator of autonomic tone. The cardiac stimulation device determines variability in one or more features derived from the evoked response in the time domain including amplitudes, slopes, integrals, time intervals, and any combination of parameters, or in the frequency domain. Frequency domain parameters include power in multiple frequency bands and ratios of power in the bands.
In accordance with one aspect, an implantable cardiac stimulation device comprises one or more pulse generators, one or more sensors, a data storage, a controller, and executable control logic that is capable of execution by the controller. The executable control logic derives at least one parameter from the sensed evoked response, determines variation of the at least one evoked response parameter over time, and derives an indicator of patient condition based on the parameter variation.